Light-emitting-diode array and method for manufacturing the same

ABSTRACT

A method for forming a light-emitting-diode (LED) array is disclosed which comprises forming a LED structure on a substrate, dividing the LED structure into at least a first and a second LED device with a gap, depositing at least one polymer material over the LED structure substantially filling the gap, removing portions of the at least one polymer material to expose a first electrode of the first LED device and a second electrode of the second LED device, and forming an interconnect on top of the at least one polymer material electrically connecting the first and second electrode.

CROSS REFERENCE

This is a divisional of application Ser. No. 12/948,504, filed on Nov.17, 2010.

BACKGROUND

The present invention relates generally to semiconductor-based lightemitting devices, and, more particularly, to a structure of such devicesand a method for manufacturing the same.

A Light-emitting diode (LED) is a semiconductor diode based lightsource. When a diode is forward biased (switched on), electrons are ableto recombine with holes within the device, releasing energy in the formof photons. This effect is called electroluminescence and the color ofthe light (corresponding to the energy of the photon) is determined bythe energy gap of the semiconductor. When used as a light source, theLED presents many advantages over incandescent light sources. Theseadvantages include lower energy consumption, longer lifetime, improvedrobustness, smaller size, faster switching, and greater durability andreliability.

FIG. 1 is a perspective view of a LED die 100 which comprises asubstrate 102, an N-type layer 110, a light-emitting layer 125 and aP-type layer 130. N-contact and p-contact 115 and 135 are formed on theN-type layer 110 and the P-type layer 130, respectively, for makingelectrical connections thereto. When a proper voltage is applied to theN- and P-contacts 115 and 135, electrons depart the N-type layer 110 andcombine with holes in the light-emitting layer 125. The electron-holecombination in the light-emitting layer 125 generates light. Sapphire isa common material for making the substrate 102. The N-type layer 110 maybe made of, for example, AlGaN doped with Si or GaN doped with Si. TheP-type layer 240 may be made of, for example, AlGaN doped with Mg or GaNdoped with Mg. The light-emitting layer 125 is typically formed by asingle quantum well or multiple quantum wells, e.g. InGaN/GaN.

In some cases, a series or parallel LED array is formed on an insulatingor highly resistive substrate (e.g. sapphire, SiC, or other III-nitridesubstrates). The individual LEDs are separated from each other bytrenches, and interconnects deposited on the array electrically connectthe contacts of the individual LEDs in the arrays. Typically, to makesure complete electrical isolation of individual LEDs, a dielectricmaterial is deposited over the LED array before the interconnectsdeposition, then patterned and removed in places to open contact holeson N-type layer and P-type layer, such that dielectric material is leftin trench between the individual LEDs on the substrate and on the mesawalls between the exposed P-type layer and N-type layer of each LED.Dielectric material may be, for example, oxides of silicon, nitrides ofsilicon, oxynitrides of silicon, aluminum oxide, or any other suitabledielectric material.

However, deposition of dielectric material is a slow and costly process.Moreover, subsequently formed interconnects which poses reliabilityconcern due to complex profiles and sharp corners of the interconnects.As such, what is desired is a system and method for manufacturing a LEDarray device cost-effectively and with improved long term reliability.

SUMMARY

A method for forming a light-emitting-diode (LED) array is disclosedwhich comprises forming a LED structure on a substrate, dividing the LEDstructure into at least a first and a second LED device with a gap,depositing at least one polymer material over the LED structuresubstantially filling the gap, removing portions of the at least onepolymer material to expose a first electrode of the first LED device anda second electrode of the second LED device, and forming an interconnecton top of the at least one polymer material electrically connecting thefirst and second electrode.

The construction and method of operation of the invention, however,together with additional objectives and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings accompanying and forming part of this specification areincluded to depict certain aspects of the invention. A clearerconception of the invention, and of the components and operation ofsystems provided with the invention, will become more readily apparentby referring to the exemplary, and therefore non-limiting, embodimentsillustrated in the drawings, wherein like reference numbers (if theyoccur in more than one view) designate the same elements. The inventionmay be better understood by reference to one or more of these drawingsin combination with the description presented herein.

FIG. 1 is a perspective view of a LED die.

FIGS. 2A and 2B are top views of an LED array in a single substrate.

FIG. 3 is a cross-sectional view of the conventional LED array shown inFIG. 2B.

FIGS. 4A-4C illustrates processing steps for forming a LED deviceaccording to an embodiment of the present invention.

FIG. 5 illustrates a trench formed in the substrate to separate two LEDdevices according to another embodiment of the present invention.

FIGS. 6A and 6B illustrate some alternative patterns of theinterconnects.

FIG. 7 illustrates a LED chip being flip mounted on a board.

DESCRIPTION

The present invention discloses a LED array structure and a processmethod for manufacturing the same. The LED array is formed by multipleLED devices for producing significant amount of light at relatively lowcurrent density. Low current density generates less heat and allowspolymer materials to be used in forming the LED array. Details of theLED array structure and the process for manufacturing the same aredescribed hereinafter.

FIGS. 2A and 2B is a top view of an of LED array 200 in a singlesubstrate 205. Referring to FIG. 2A and for illustration purpose, theLED array 200 has four rows (Y) and four columns (X) of separated yetidentical LED devices 210[0:3, 0:3], each shaped like a mesa. The LEDdevices 210 may be separated by laser etching or Inductively CoupledPlasma Reactive Ion Etching (ICP-RIE). As an example, neighboring LEDdevices, 210[2, 3] and 210[3, 3] are separated by a gap 220[2]. The LEDdevice 210[2, 3] has two electrodes, i.e., pads 213[2, 3] and 215[2, 3]serving as anode and a cathode of the LED device 210[2, 3],respectively. The electrodes can be formed on P-GaN and N-GaN (eitherP-side up or N-side up). One LED device's anode pad is placed close to aneighboring LED device's cathode pad, so that the LED devices 210 can beeasily connected in series.

Referring now to FIG. 2B, the pad 213[2, 3] and the pad 215[3, 3] areconnected by an interconnect 230[2, 3]. The pads 213 and 215 aretypically formed by a metal, and so is the interconnect 230. The pads213 and 215 and the interconnect 230 may not necessarily be formed bythe same metal.

FIG. 3 is a cross-sectional view of the conventional LED array 202 at alocation A-A′ shown in FIG. 2B. On a single substrate 205, multiple LEDdevices 210 are built with cross-sections of two adjacent ones, 210[1,3] and 210[2, 3] shown in FIG. 3. The pad 213[1, 3], for instance, is ananode of the LED device 210[1, 3]. The pad 215[2, 3] is a cathode of theLED device 210[2, 3]. Conventionally, an oxide layer 310 is deposited inthe gap 220[1] between the LED devices 210 to electrically isolate thepads 213 and 215 from adjacent structures. Then the metal interconnect230[1, 3] is formed on top of the oxide layer 310 to connect the pads213[1, 3] and 215[2, 3]. Due to the depth of gap 220, the oxide layer310 cannot fill up the gap 220, and causing the metal interconnect 230to form a complex profile with sharp corners. The sharp corners arerelatively easy to break hence become a reliability concern.

FIGS. 4A-4C illustrates processing steps that uses a polymer to fill upthe gap 220 between the LED devices 210 according to an embodiment ofthe present invention. Because the LED devices in accordance with thepresent invention are intended to be used at high efficiency with littleheat generated, it is feasible to leave polymer material in a finishedLED device.

Referring to FIG. 4A, after each individual LED devices 210 andrespective pads 213 and 215 are formed, a polymer layer 410 is depositedover the LED devices 210. The polymer layer 410 fills up the gap 220.Photoresist, such as polymethylglutarimide (PMGI) or SU-8, is apreferred polymer material. Refractive index of the polymer layer 410ranges from 1 to 2.6 (between air and semiconductor) to enhance lightextraction. Optical transparency of the polymer layer 410 is equal to ormore than 90%, and preferably equal to or more than 99%. Typically, athickness of the polymer layer 410 measured on top of the pad 213 isapproximately 2 micron meter. The polymer layer 410 can be pre-mixedwith phosphor (about 30 weight percentage loading) to adjust the outputlight color. However, the relative dimension between polymer coatingthickness and phosphor particle size should be coordinated. For example,when a thickness of the polymer layer 410 at the pad 213 is about 3micron meter, proper phosphor particle size is approximately 3 micronmeter or less.

Referring to FIG. 4B, a patterned mask 420 is applied over the polymerlayer 410. The mask 420 has openings 423 at the locations of pads 213and 215 to allow the removal of the polymer layer 410 thereon. Thepolymer removal process also smooth out surface profile of the polymerlayer 410.

After the polymer removal process and pads 213 and 215 being exposed, asurface hydrophilic modification is performed on the polymer surface(e.g., oxygen plasma) to transform the originally hydrophobic surfaceinto hydrophilic surface. Therefore, a subsequently formed metal-basedinterconnect can have improved adhesion to the polymer layer 410.

Referring to FIG. 4C, a interconnect 430 is then formed on top of thepolymer layer 410 to connect the pad 213 and pad 215. Because of thesmooth surface profile of the polymer layer 410, the subsequently formedmetal-based interconnect 430 can have thin and smooth profile withimproved endurance. In comparison, conventional interconnect (230 inFIG. 3) is easy to brake due to complex profiles and sharp corners. Eventhough the fragileness of the conventional interconnect 230 can beslightly improved by increasing the thickness of the interconnect 230,this is done at increased cost due to both additional material used andadditional processing time.

In the present invention, as mentioned above, the LED devices 210 areintended to be used at high efficiency with little heat generated,metals with lower melting points, such as Al, In, Sn or related alloymetals, can be used to form the major component of interconnect 430(equal to or more than 90 vol %), which further lowers the cost ofproducing the LED device. Fabrication processes, such as chemical vapordeposition, sputtering or evaporation of the metal can be used forforming the interconnect 430. In an exemplary process, three layers ofmetal, Ti/Al/Pt, are sputtered to form the interconnect 430.

Furthermore, mixture of metal powder and polymer (e.g. silver paste) canalso be used to form the interconnect 430. Corresponding fabricationprocess may be screen printing or stencil printing process with evenlower manufacture cost.

In addition, the smoothness of the polymer layer 410 allows sizes of thepads 213 and 215 and interconnect 430 to be smaller than theconventional ones shown in FIG. 3, so that less LED area will beshielded by the opaque pads 213 and 215 and interconnects 430.

In addition to the aforementioned providing a smooth surface, thepolymer layer 410 can also absorb and dissipate heat from neighboringLED devices 210, especially when the polymer layer 410 is mixed withsome special materials such as ceramics and carbon-based nanostructures.

Ceramics and carbon-based nanostructures absorb heat energy and emit itas far-infrared wavelength energy. Infrared radiation is a form ofelectromagnetic radiation with wavelengths longer than those at thered-end of the visible portion of the electromagnetic spectrum butshorter than microwave radiation. This wavelength range spans roughly 1to several hundred microns, and is loosely subdivided—no standarddefinition exists—into near-infrared (0.7-1.5 microns), mid-infrared(1.5-5 microns) and the far-infrared (5 to 1000 microns).

Ceramics which are inorganic oxides, nitrides, or carbides areconsidered as the most effective far infrared ray emitting bodies. Anumber of studies on ceramic far infrared ray emitting bodies have beenreported, including zirconia, titania, alumina, zinc oxides, siliconoxides, boron nitride and silicon carbides. Oxides of transitionelements such as MnO2, Fe2O3, CuO, CoO, and the like are considered moreeffective far infrared ray emitting bodies. Other far infrared rayemitting body includes carbon-based nanostructures, such as carbonnanocapsule and carbon nanotubes. They also show a high degree ofradiation activity. These materials are very close to a black bodyexhibiting a high degree of radiation activity throughout the entireinfrared range. In accordance with an embodiment of the presentinvention, the polymer layer 410 is pre-mixed with ceramics orcarbon-based nanostructures which absorb the heat from nearby LEDdevices 210 and/or phosphors, and then dissipate the heat as farinfrared radiation. This characteristic can be used to allow heat toescape from the LED devices 210 even when the LED devices 210 are in asealed enclosure without heat sinks or cooling fans. Of course, with theaddition of heat sinks or cooling fans heat can be better dissipated.

FIG. 5 illustrates a trench 502 formed in the substrate 205 to separatetwo LED devices according to another embodiment of the presentinvention. The trench 502 is typically laser etched into the substrateduring the formation of the gap between two LED devices 210 in order toallow more light to come out the lateral sides of the LED devices 210.As a result, light extraction efficiency of a whole LED chip thatincorporates an array of the LED devices 210 will be increased. Thedeeper the trench 502 is, the higher the light extraction efficiency theLED chip attains. Typically, a depth of the trench 502 measured from anoriginal surface of the substrate 205 to the bottom of the trench 502 iscontrolled at a range between 20 microns and 100 microns.

However, the trench 502 is more difficult to fill. As shown in FIG. 5, aPMGI layer 510 is first deposited in the trench 502, and then followedby a SU-8 layer 520 in accordance with the embodiment of the presentinvention. The PMGI layer 510 has better filling characteristic. TheSU-8 layer 520 deposited on top of the PMGI layer 510 also serves as abarrier layer protecting the underneath PMGI layer 510 from reactingwith developers in subsequent photoresist processes. One of suchphotoresist processes is for forming the interconnect 430 by metalsputtering in which a NR-7 patterning photoresist is used. The developerused with the NR-7 photoresist can react with the PMGI layer 510 if notfor the protection of the SU-8 layer 520. However, if the interconnect430 is formed by a silver paste in a printing process, a single PMGIlayer can be used for filling the entire gap, including the trench 502,between the two LED device 210 for further saving processing cost.

FIGS. 6A and 6B illustrate some alternative patterns of the interconnect430. Referring to FIG. 6A, interconnects 630 a and 630 b are moved toedges of the LED devices 210 corresponding to relocations of electrodepads (not shown). Referring to FIG. 6B, interconnects 635 a and 635 bare T-shaped to connect neighboring LED devices 210. Varying theinterconnect patterns is to reduce areas of the interconnects, so thatless light generated by the LED devices is shielded by theinterconnects.

FIG. 7 illustrates a LED chip 702 being flip mounted on a board 720. TheLED chip 702 is produced through the processes shown in FIGS. 4A˜4C,i.e., a plurality of the LED devices 210 are formed on the samesubstrate 205 (not shown in FIG. 7). When the substrate 205 is asapphire which is highly transparent to light, the LED chip 702 can beflip mounted on a board 720. In such case, the substrate 205 of the LEDchip 702 is on the top, the plurality of the LED devices 210 are belowthe substrate 205. Before the LED chip 702 being flip mounted on theboard 720, solder balls 710 are first formed on the terminals of the LEDchip 702. Then the LED chip 702 is flipped over and placed on the board720 with the solder balls 710 aligned to corresponding terminalinterconnects 722. After a melting process, the solder balls 710 bondsthe LED chip 702 to the board 720 through the terminal interconnects722. Apparently, the flip-chip technology yields the shortestboard-level interconnects and better electrical characteristics. Whenmultiple LED chips 702 are mounted on the same board 720, mountingdensity for the flip-chip mounting can be higher than conventional wirebonding. In addition, after the LED chip 702 being flip mounted on theboard 720, the substrate (not shown in FIG. 7) on which the LED chip 702is grown can be removed for even better light emission.

The above illustration provides many different embodiments orembodiments for implementing different features of the invention.Specific embodiments of components and processes are described to helpclarify the invention. These are, of course, merely embodiments and arenot intended to limit the invention from that described in the claims.

Although the invention is illustrated and described herein as embodiedin one or more specific examples, it is nevertheless not intended to belimited to the details shown, since various modifications and structuralchanges may be made therein without departing from the spirit of theinvention and within the scope and range of equivalents of the claims.Accordingly, it is appropriate that the appended claims be construedbroadly and in a manner consistent with the scope of the invention, asset forth in the following claims.

1. A method for forming a light-emitting-diode (LED) array, the methodcomprising: forming a LED structure on a substrate; dividing the LEDstructure into at least a first and a second LED device with a gap;depositing at least one polymer material over the LED structuresubstantially filling the gap; removing portions of the at least onepolymer material to expose a first electrode of the first LED device anda second electrode of the second LED device; and forming an interconnecton top of the at least one polymer material electrically connecting thefirst and second electrode.
 2. The method of claim 1, wherein the atleast one polymer material is a photoresist.
 3. The method of claim 2,wherein the material of the photoresist is selected from the groupconsisting of PMGI and SU-8.
 4. The method of claim 1, wherein theoptical transparency of the polymer material is equal to or more than90%.
 5. The method of claim 1, wherein the refractive index of thepolymer material ranges from 1 to 2.6 for enhancing light extraction. 6.The method of claim 1, wherein the polymer material is pre-mixed withphosphor.
 7. The method of claim 1, wherein the polymer material ispre-mixed with an infrared radiating material.
 8. The method of claim 7,wherein the infrared radiating material is selected from the groupconsisting of a ceramic and a carbon-based nanostructure.
 9. The methodof claim 1 further comprising performing a surface hydrophilicmodification on a polymer surface to transform the originallyhydrophobic surface into hydrophilic surface.
 10. The method of claim 1further comprising applying a patterned photo mask for selectivelyremoving the portions of the polymer material.
 11. The method of claim1, wherein the gap is created by laser etching or Inductively CoupledPlasma Reactive Ion Etching (ICP-RIE).
 12. The method of claim 1,wherein a bottom of the gap reaches beneath an original surface of thesubstrate.
 13. The method of claim 12, wherein a distance between thebottom of the gap and the original surface of the substrate ranges from20 microns to 100 microns.
 14. The method of claim 1, wherein the gap issubstantially filled with two or more polymer materials forming amulti-layered structure.
 15. The method of claim 1 further comprisingproviding a board for flip mounting the LED array thereon with thesubstrate being above the first and second LED devices.
 16. The methodof claim 15, wherein the substrate is removed after the LED array isflip mounted on the board.